The present invention concerns a method for inspecting welds between pipe sections when laying pipelines. More specifically, but not exclusively, this invention relates to a method for the non-destructive inspection of welds between pipe sections using a phased ultrasonic transducer array system.
The present invention is particularly, but not exclusively, concerned with a non-destructive weld testing method used when laying oil or gas pipelines underwater. When laying a pipeline at sea it is customary to weld, on a lay-barge, individual pipe sections to a pipe string (the pipe string leading towards the seabed). The welding process takes place on the lay-barge. The pipe-string, when being laid, is under great tension and weld joints must be sufficiently strong to withstand the high forces imposed on them. As such, each time a pipe is welded to another pipe, extensive tests are made to ensure that the quality of the weld joint formed is sufficient.
It is customary to use non-destructive testing (NDT) techniques to inspect the quality of welds between pipe sections. International standards for NDT methods used in laying pipelines place strict limits on the accuracy of these methods. In order to assess whether a given weld is of sufficient quality, it is important that the NOT method used can reliably detect and provide accurate information about the size and location of any defects found in the weld. The time pressure that is present when laying pipelines at sea also means that it is important that any defects in the welds be detected and analysed very quickly.
Ultrasonic testing is a popular NDT technique for inspecting welds. Typically, ultrasonic sound waves are used to detect defects in the region of the weld. For most applications, shear waves (also called transverse waves) are used to detect and gauge the size of defects. Compression waves (longitudinal waves) are generally less suitable because the smallest size of defect detectable by means of compression waves is typically twice as large as the smallest size of defect detectable by means of shear waves in an otherwise identical set-up. Certain pipeline applications, however, use pipes internally clad with corrosion resistant alloy (CRA) materials such as austenitic stainless steel. The weld material in weld joints in such pipelines is also typically of austenitic material. Such austenitic materials have a coarse granular structure that skews and attenuates ultrasonic waves. Longitudinal waves are affected less than shear waves and are thus preferred for inspection. Welds in which the material welded together and/or the material in the heat-affected zone and/or the weld material (the filler material) itself comprises austenitic material will hereby be referred to as austenitic welds. Such ultrasonic testing is generally supplemented with other non-destructive tests to ensure reliability.
WO2005/045418 discloses a method for the ultrasonic inspection of an austenitic weld between two pipes. It uses two sets of ultrasound transducers placed either side of the weld. Accurate location and sizing of defects in spite of the skewing and attenuating properties of the weld material is achieved by using three different ultrasound beams. Firstly, two differently angled longitudinal beams are directed towards a given region. The detection and analysis of the ultrasound waves that result allow for detection of a defect. The size of the defect may then, optionally, be gauged by means of a third, shear-wave, beam of ultrasound radiation. In order to inspect different regions of the weld, the transducers need to be moved or different additional transducers used. It would appear that if the same transducers are used to inspect different locations then the transducers need to be moved manually. To perform NOT on a pipe weld with the apparatus of WO2005/045418, it would seem necessary either to realign the transducers for different locations and/or provide a large number of transducers.
The present invention seeks to mitigate one or more of the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved method for non-destructively testing a weld between two sections of metal pipe during the laying of a pipeline.